1. Field of the Invention
The present invention relates to an improved screen printing machine.
2. Description of the Prior Art
In screen printing a screen made of fine-meshed textile is stretched in taut condition, clamped along all sides, in a frame of shallow box-like build, and a pattern of resist is coated, either manually or optochemically, in the screen. The screen is placed on top of the surface to be printed and ink is applied to the upper face of the screen by means of a squeegee. Thus, pressure applied by the squeegee to the resist-coated screen face causes some of the ink to soak through its pervious or non-coated areas so that a pattern of ink is printed in the surface below. A typical screen printing machine is shown in FIG. 1.
Screen printing machines operate with a variety of inks and reproduce the image or pattern in a relatively thick layer of ink so that the resultant print can be highly resistant to chemicals and have enhanced weatherproof properties. Mainly because of these advantages, screen printing in recent years has come to be used in the production of circuit patterns in integrated circuit (IC) boards.
If the squeegee is moved in the face of the screen, with minimum pressure enough to keep the inking tip of the squeegee in uniform contact with the screen but just short of causing any tension in the screen, the locus described by the squeegee tip is in theory a definite segment of an ellipse cut at both ends by the distance traveled by the squeegee (as depicted by dotted line in FIG. 2). The locus is also a path that is followed by the moving bottom P of the screen where it is caused to slightly elastically bend down under the foregoing minimum pressure in the squeegee. To make the print in actual practice of screen printing, however, the squeegee has to be moved with some required measure of pressure for ink transfer, naturally greater than the above minimum pressure, so that the underside of the screen is brought into close contact with the surface to be printed, all the way from end to end. This means that the moving bottom P of the screen follows a linear path, not a curve similar to the dotted line in FIG. 2, since the surface to be printed is generally flat and evenly spaced from the horizontal plane of the screen. Thus, with the movement of the squeegee, the screen is subjected to a varying degree of tension depending on its relative location. In more detail, where the screen is away from its middle in the direction of travel of the squeegee, the tension becomes increasingly greater since the screen must sag far down beyond the theoretical curve (dotted line in FIG. 2) at both ends thereof. As a consequence, the screen has greater holes in the mesh at both ends than in the middle, so that more ink gets through there. Thus, the resultant printed pattern or image is likely to have a local distortion or dimensional deviation from the original pattern of the resist. This is a problem with most conventional screen printing machines.
The prior art also has other printing problems. In actual operation, the squeegee is normally moved at a constant angle with respect to the plane of the surface to be printed, with its inking tip being pressed against the surface over the screen. Thus, as shown in FIG. 2, the screen sags at point P where the bottom side of the screen 3 forms angles .alpha. and .beta. with respect to the plane of the surface 4. Angle .beta. is the angle formed on the side of the squeegee where it carries an inking edge at its lower tip. The opposite angle .alpha. is generally known as the snap-off angle. Because of the designs of those prior art devices, squeegee movement along the effective screen length results in a change in the snap-off and opposite angles .alpha. and .beta., as illustrated in FIG. 2 showing that the snap-off angle .alpha. gets larger as the squeegee moves from P1 to P2. This angular change brings the screen 3 to contact the surface 4 without uniformity, with a resultant uneven transfer of ink. Thus, the resultant reproduced print has tended to occur in a varying degree of ink thickness.
Therefore, in order to accomplish faithful pattern reproduction in screen printing, it is necessary to maintain not only the snap-off angle but also the tension in the screen under the pressure of the moving squeege uniform or nearly uniform.
To keep a uniform tension in the screen during the movement of the squeege is to maintain the length of the screen constant.
In addition, what is more ideal from the viewpoint of practical operation is to maintain the foregoing angle .beta. uniform, along with the snap-off angle.